U.S. patent number 5,040,175 [Application Number 07/507,558] was granted by the patent office on 1991-08-13 for wireless information transmission system.
This patent grant is currently assigned to Inland Steel Company, NCR Corporation. Invention is credited to Michael A. Masleid, Bruce T. Tuch.
United States Patent |
5,040,175 |
Tuch , et al. |
August 13, 1991 |
**Please see images for:
( Certificate of Correction ) ** |
Wireless information transmission system
Abstract
A wireless local area network (10) includes a plurality of
stations (12) adapted to transmit and receive message packets via a
distribution system (30). In a time division multiplexing
embodiment the network (10) operates using successive timing frames
(FR1, FR2, etc.) each consisting of three timing intervals (T1, T2,
T3). During the first timing interval (T1) the distribution system
(30) normally transmits synchronizing packets (HBT), to which a
station (12) desiring to transmit responds by transmitting an
information packet during the next second timing interval (T2).
During the subsequent first timing interval (T1), the distribution
system (30) retransmits the received information packet. Any
transmitting station (12) which detects an information packet
mismatch transmits a collision reporting code (CDT) during the next
third timing interval (T3) to which the distribution system (30)
responds by transmitting a collision acknowledgement code (CAKT) to
all the stations (12) thereby aborting information transmission. A
frequency division multiplexing embodiment is also disclosed.
Inventors: |
Tuch; Bruce T. (Ce De Bilt,
NL), Masleid; Michael A. (Dyer, IN) |
Assignee: |
NCR Corporation (Dayton,
OH)
Inland Steel Company (Chicago, IL)
|
Family
ID: |
24019124 |
Appl.
No.: |
07/507,558 |
Filed: |
April 11, 1990 |
Current U.S.
Class: |
370/245; 714/750;
370/345; 370/350; 370/445; 370/349 |
Current CPC
Class: |
H04L
1/14 (20130101); H04L 12/413 (20130101); H04W
24/00 (20130101); H04W 84/12 (20130101); H04W
48/08 (20130101) |
Current International
Class: |
H04L
1/12 (20060101); H04L 1/14 (20060101); H04L
12/407 (20060101); H04L 12/413 (20060101); H04L
12/28 (20060101); H04J 003/02 () |
Field of
Search: |
;340/825.5 ;371/34,33,32
;370/85.2,85.3,94.1,94.2 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0184383 |
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Nov 1985 |
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EP |
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0286614 |
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Mar 1988 |
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EP |
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0294133 |
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May 1988 |
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EP |
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2206468 |
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Jun 1988 |
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EP |
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Primary Examiner: Olms; Douglas W.
Assistant Examiner: Ton; Dang
Attorney, Agent or Firm: Hawk, Jr.; Wilbert Jewett; Stephen
F. Sessler, Jr.; Albert L.
Claims
What is claimed is:
1. A method of transmitting information packets between a plurality
of stations in a wireless local area network, comprising the
following steps:
(a) providing a plurality of local area network stations;
(b) providing a distribution system for receiving packets
transmitted by all said local area network stations and for
transmitting packets to all said local area network stations;
(c) defining first, second and third wireless transmission
channels, said first wireless transmission channel for transmitting
packets from said distribution system to said local area network
stations, and said second and third wireless transmission channels
for transmitting packets from said local area network stations to
said distribution system;
(d) defining a transmission-enabling condition using said first
wireless transmission channel;
(e) transmitting from a local area network station which has a
message to transmit, an information packet over said second
wireless transmission channel in response to a
transmission-enabling condition;
(f) receiving a transmitted information packet at said distribution
system;
(g) retransmitting the received information packet from aid
distribution system over said first wireless transmission
channel;
(h) comparing at any local area network station which has
transmitted, the received retransmitted information packet with the
information packet originally transmitted by that local area
network station;
(i) in the event of a comparison match, transmitting a further
information packet over said second wireless transmission
channel;
(j) in the event of a comparison mismatch, transmitting a collision
reporting packet from such local area network station over said
third wireless transmission channel;
(k) receiving a transmitted collision reporting packet at said
distribution system;
(l) transmitting a collision acknowledgment condition, in lieu of
the information received over the second transmission channel, from
said distribution system over said first wireless transmission
channel;
(m) terminating message transmission at all local area network
stations which are transmitting, in response to said collision
acknowledgment condition; and
(n) returning to step (d).
2. A method according to claim 1, in which said first, second and
third wireless transmission channels are formed by first, second
and third timing intervals in repetitive timing frames; in which
said defining step (d) includes the step of: transmitting from said
distribution system during said first timing intervals,
synchronizing packets and receiving said synchronizing packets at
said local area network stations; and in which said transmitting
step includes transmitting an information packet during one of said
second timing intervals, in response to receipt of one of said
synchronizing packets.
3. A method according to claim 2, in which said retransmitting step
(g) includes the step of retransmitting the received information
packet from said distribution system during the next first timing
interval instead of synchronizing packet.
4. A method according to claim 3, in which said terminating step
(m) includes the steps of detecting at said distribution system
whether a transmission signal is present during said third timing
intervals; in response to the detection of a transmission signal
during a third timing interval, transmitting a collision
acknowledgment code during the next first timing interval instead
of a synchronizing packet; and terminating message transmission
from all transmitting stations in response to receipt of said
collision acknowledgement code.
5. A method according to claim 1, in which said collision reporting
packet includes a representation of the address of the local area
network station transmitting said collision reporting packet.
6. A method according to claim 5, in which said representation is a
scrambled version of the station address.
7. A method according to claim 4, in which in said step (f), if an
indecipherable information packet is received, said distribution
system responds by transmitting said collision acknowledgement code
during the next first timing interval.
8. A method according to claim 1, in which said first, second and
third wireless transmission channels are formed by first, second
and third transmission frequencies; in which said defining step (c)
includes detecting the absence of a carrier signal on said first
wireless transmission channel; and in which said collision
reporting packet causes said distribution system to transmit a
collision acknowledgement condition over said first wireless
transmission channel.
9. A method according to claim 8, in which in said step (f), if
indecipherable information is received, said distribution system
responds by transmitting said collision acknowledgement code over
said first wireless transmission channel.
Description
BACKGROUND OF THE INVENTION
This invention relates to a method of transmitting information
between a plurality of stations in a local area network.
Various methods are known for sharing network communication
channels among stations in a local area network (LAN). One widely
used method is known as CSMA/CD, (carrier sense multiple
access/collision detect). According to this known method, a station
wishing to transmit a message listens until the transmission
channel is idle before commencing to transmit an information
packet. Furthermore, the station continues to listen to the channel
after commencing a transmission and if a collision is detected,
that is, more than one station has commenced to transmit an
information packet, any station which detects such collision
terminates its message transmission and transmits a burst of noise
(or garble) such that all other stations can be informed of the
collision, terminate message transmission, and wait a random time
before attempting to commence a further message transmission.
From Patent Abstracts of Japan volume E-690, page 102, abstract of
Japanese Published Application No. 63-187746, there is known a
wireless communication system wherein a plurality of stations
communicate via a repeater installation. A first frequency is used
for transmission from the stations to the repeater installation and
a second frequency is used for transmissions from the repeater
installation to the stations. If the repeater installation detects
a collision on the first frequency transmissions, then it
immediately informs all stations of such collision using the second
frequency, such that the stations can take appropriate action.
Although the CSMA/CD method has proved satisfactory for
transmissions over a cable physical medium, it has been found that
a problem arises if it is attempted to apply the method to wireless
radio transmissions in that small radio signals may be totally
masked by larger signals during reception. This phenomenon is known
as the "capture effect", and may prevent the detection of
collisions in a system such as that disclosed in the aforementioned
Japanese patent abstract.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method of
transmitting information between a plurality of stations in a local
area network, which alleviates the problem of collision detection
when a wireless radio communication medium is utilized.
Therefore, according to the present invention, there is provided a
method of transmitting information between a plurality of stations
in a local area network, comprising the following steps:
(a) providing a distribution system;
(b) defining first, second and third transmission channels;
(c) defining a transmission-enabling condition using said first
transmission channel;
(d) transmitting from a station which has a message to transmit, an
information packet over said second transmission channel in
response to a transmission-enabling condition;
(e) receiving a transmitted information packet at said distribution
system;
(f) retransmitting the received information packet from said
distribution system over said first transmission channel;
(g) comparing at any station which has transmitted, the received
retransmitted information packet with the information packet
originally transmitted by that station;
(h) in the event of a comparison match, transmitting a further
information packet over said second transmission channel;
(i) in the event of a comparison mismatch, transmitting a collision
reporting packet from such station over said third transmission
channel;
(j) receiving a transmitted collision reporting packet at said
distribution system;
(k) transmitting a collision acknowledgement condition, in lieu of
the information received over the second transmission channel, from
said distribution system over said first transmission channel;
(1) terminating message transmission at all stations which are
transmitting, in response to said collision acknowledgement
condition; and
(m) returning to step (c).
It will be appreciated that by utilizing a method according to the
invention, each station, by virtue of the comparison step, can
detect that its own transmission is invalid, that is, not received
at the distribution system. Also, any station which has "captured"
the transmission channel can be informed, by means of the collision
acknowledgement condition that its transmitted packet has collided
with the transmitted packet of another station.
BRIEF DESCRIPTION OF THE DRAWINGS
Two embodiments of the present invention will now be described by
way of example, with reference to the accompanying drawings, in
which:
FIG. 1 shows a local area network using wireless radio
communication and including a plurality of stations and a central
distribution system, operating according to a time division
multiplexing scheme;
FIG. 2 is a block diagram of a typical station;
FIG. 3 is a block diagram of the distribution system;
FIGS. 4A to 4D inclusive are diagrams showing the format of message
packets which are transmitted over the network;
FIG. 5 is a timing diagram of the operation of the network when no
collision occurs;
FIG. 6 is a timing diagram of the operation of the network when a
collision occurs;
FIG. 7 is a block diagram of a wireless radio local area network
operating according to a frequency division multiplexing
scheme.
DETAILED DESCRIPTION
Referring first to the time division multiplexing embodiment shown
in FIG. 1, there is shown a wireless information local area network
(LAN) 10, which uses three transmission channels defined by timing
intervals T1, T2, T3, to be explained hereinafter.
The network 10 includes a plurality of stations 12, identified
individually as stations 12-1, 12-2, ..., 12-N, among which
information is to be transmitted. Each station 12 includes a
transceiver 14 and a user device 16, identified individually as
transceivers 14-1, 14-2, ..., 14-N and user devices 16-1, 16-2,
..., 16-N, respectively. The user devices 16 are coupled to the
respective transceivers 14 by output and input lines 18, 20
identified individually as output lines 18-1, 18-2, ..., 18-N and
input lines 20-1, 20-2, ..., 20-N.
The transceivers transmit and receive radio frequency (RF) signals
via antennas 22, identified individually as antennas 22-1, 22-2,
..., 22-N. The stations 12 may be for example personal computers,
printers, data terminals or data handling devices.
The network 10 also includes a distribution system 30 which has an
antenna 32, and is adapted to receive messages transmitted from a
transmitting station 12 and retransmit the received messages such
that the messages are received by all the stations 12 including, of
course, the station to which the messages are addressed. All
transmissions take place over the same frequency channel.
Referring now to FIG. 2 there is shown a block diagram of a typical
station transceiver 14, which can be any one of transceivers 14-1,
14-2, ..., 14-N shown in FIG. 1. The transceiver 14 includes a data
buffer 40 which receives input data to be transmitted, from the
associated user device 16 (FIG. 1). The transceiver antenna 22 is
coupled via line 42 to an RF receiver 44 which provides demodulated
data on an output line 46 connected to a data handling unit 48,
which also receives an input over a line 50 from the data buffer
40. The receiver 44 also provides a carrier detect signal on a line
51 in response to detection of a carrier signal. The data handling
unit 48 is coupled via lines 54 and 56 to a timing control circuit
52. The data handling unit 48 provides decoding, buffer storage and
comparison capabilities as will be discussed hereinafter. The
timing control circuit 52 controls the timing operations of the
data handling unit 48. The data handling unit 48 has an output line
58 connected to an RF transmitter 60 which modulates the applied
data and is coupled over a line 62 to the antenna 22. The data
handling unit 48 also has an output connected to the line 20 which
is coupled to the associated user device 16.
Referring now to FIG. 3, there is shown a block diagram of the
distribution system 30. The antenna 32 is connected over a line 70
to an RF receiver 72 which demodulates the received signals, and
has a data output line 74 and a carrier detect output line 75, both
coupled to a data handling unit 76 which is coupled via lines 78
and 80 to a timing control circuit 82. The data handling unit 76
provides decoding and buffer storage capabilities as will be
discussed hereinafter. The timing control circuit 82 includes timer
devices for defining the timing intervals T1, T2, T3, discussed
subsequently. The data handling unit 76 has an output line 84
coupled to an RF transmitter 86 which modulates the applied data
and is coupled over a line 88 to the antenna 32.
Information is transmitted in the network 10 in the form of
packets. The network is synchronized under the control of
synchronizing packets, referred to as heartbeat packets HBT, which
are transmitted by the distribution system 30 at regular intervals
defining first timing intervals T1, of successive timing frames
FR1, FR2, etc. The heartbeat packets HBT are received by all the
stations 12 and are recognized as heartbeat packets HBT by decoders
(not shown) in the respective data handling units 48 in the
stations 12. Referring to FIG. 4A, there is shown the format of a
heartbeat packet 100 transmitted on the network 10. The heartbeat
packet 100 includes a preamble (PR) portion 102, a start delimiter
portion 104, and identification code (HBT) portion 106, and an end
delimiter portion 108. As is well known in the art, the preamble
portion 102 permits the receivers 44 (FIG. 2) to adjust the control
variables, such as gain and to ensure bit synchronization.
Referring now to FIG. 5, there is shown a timing diagram for the
operation of the network 10 when no collisions occur. In the first
timing interval T1 of the first timing frame FR1, an HBT packet 110
is transmitted by the distribution system 30 and received by all
the stations 12. Since it is assumed that at this time, no station
12 desires to transmit information, the remaining timing intervals
T2 and T3 of the first timing frame do not contain any
transmissions. In the next timing frame FR2, a further HBT packet
112 is transmitted. It is assumed that station No. 1 desires to
transmit a message at this time. Consequently, after recognition of
the end delimiter portion of the HBT packet the timing control unit
52 (FIG. 2) in station No. 1 causes the data handling unit 48 to
transmit a first message portion from the data buffer 40, formatted
into an information packet (IAl) 114. Referring briefly to FIG. 4B,
there is shown the format of a typical information packet 120. The
information packet contains a preamble portion 122, a start
delimiter portion 124, an identification code (IC) portion 126, and
an end delimiter portion 134. The destination address (DA) portion
128, the source address (8A) portion 130 and the data portion 132
are directly taken from the message delivered by the user device 16
over the output line 18. The illustrated format in FIG. 4B is only
applicable to the first packet of a message. All subsequent packets
of the message contain a data portion but no address portions.
Returning now to FIG. 5, the information packet 114, transmitted by
station No. 1 during time interval T2 of timing frame FR2 is
received by the distribution system 30 and temporarily stored in a
buffer (not shown) included in the data handling unit 76. During
the first timing interval Tl of the timing frame FR3 an information
packet (IAl) 140, containing the information just received, is
transmitted by the distribution system 30 instead of a heartbeat
packet. All the stations 12 recognize the packet as an information
packet, by decoding of the identification code portion thereof in
the data handling units 48 of the stations, and maintain timing
frame synchronization with the information packet 140, in the same
manner as with a heartbeat packet HBT.
However, no station 12 may commence a message transmission in
response to receiving an information packet, as opposed to a
heartbeat packet. Thus, during timing interval T2 of timing frame
FR3, only the already transmitting station No. 1, may transmit. The
data handling units 48 in the other stations no longer see a
transmission enabling condition and are consequently inhibited from
commencing a message transmission during timing interval T2 of
timing frame FR3.
Thus, during the next timing interval T2 of the timing frame FR3,
the transmitting station No. 1 transmits a further portion of its
message in the form of an information packet (IA2) 142. This
information packet (IA2) is then retransmitted as packet 144 during
the first timing interval Tl of timing frame FR4. This procedure
continues, as indicated by packets IA3, IA4 in FIG. 5 until the
entire message has been transmitted by station No. 1 and received
by its destination station whereafter the distribution system 30
again transmits heartbeat (HBT) packets to maintain network
synchronization.
The above described operation assumes that only one station 12,
station No. 1 in the particular example, attempts to transmit
during the timing interval T2 of timing frame FR2. The operation
will now be considered where more than one station transmits during
such timing interval. Referring to FIG. 6, it is assumed that
during the first timing interval Tl of timing frame FR1 a heartbeat
packet (HBT) 150 is transmitted by the distribution system 30 but
that none of the stations 12 desires to transmit information at
this time. Consequently, no information is transmitted during
timing interval T2 of timing frame FRl. During timing interval Tl
of timing frame FR2, another heartbeat packet 152 is transmitted by
the distribution system 30, and received by all the stations 12. It
is now assumed that (at least) two stations 12, identified in FIG.
6 as station No. 1 and station No. 3, desire to transmit
information. Consequently, during timing interval T2 of timing
frame FR2, station No. 1 transmits an information packet (IAl) 154
and station No. 3 transmits an information packet (IBl) 156.
During timing interval T2 of timing frame FR2, there are two
possible reception conditions at the distribution system 30. The
first reception condition is that, due to the aforementioned
"capture" effect, the signals of one of the transmitted information
packets are totally masked by the signals of the other information
packet. For example, assume that the information packet (IAl) 154
transmitted by station No. 1 is received by the distribution system
30 and totally masks the information packet (IBl) 156 transmitted
by station No. 3. Thus, the information packet 156 is not received
by the data handling unit 76 of the distribution system 30. The
distribution system 30 therefore transmits, during timing interval
Tl of timing frame FR3, the information packet IAl, as shown by
reference 158. As described with reference to FIG. 5, the
information packet IAl is received by all the stations 12.
Furthermore, the data handling units 48 of the transmitting
stations No. 1 and No. 3 compare the received information packet
158 with the transmitted information packets 154 and 156 which are
stored in a buffer register (not shown) in the respective data
handling unit 48. The comparison at the transmitting station No. 1
results in a match, since the information packets 154 and 158 are
identical. Station No. 1 transmits a further information packet
(IA2) 160 during timing interval T2 of timing frame FR3. However,
the comparison at the transmitting station No. 3 results in a
mismatch, since at least the source address portions of the
information packets IAl and IBl are different i.e. station No. 3
does not recognize its address as the source address in the
received information packet IAl. This mismatch serves as a local
"collision detect" i.e. simultaneous transmissions by two stations
12 are detected by station No. 3. In response to the comparison
mismatch, the data handling unit 48 of station No. 3 transmits a
collision reporting packet (CDT) 162 during timing interval T3 of
timing frame FR3, and reports a detected collision to the user
device 16 of station No. 3 over the line 20. A random timer (not
shown) included in the user device 16 of station No. 3 is
triggered, and a retry is initiated when the random timer times
out.
Referring briefly to FIG. 4C, there is shown the format of a
collision reporting packet 170. The collision reporting packet 170
includes a preamble portion 172, a start delimiter position 174, a
collision detect (CDT) portion 176, a source address portion (SA)
178, containing the address of the station which detected the
mismatch, and an end delimiter portion 180. The inclusion of the
source address portion (SA) 178 in the collision reporting packet
170 has the advantage that, if two or more stations are reporting
collisions, total signal cancellation is not possible, since the
packets are not identical. In an alternative arrangement, instead
of the source address in the portion 178, the collision reporting
packet could contain a scrambled version of the station address,
obtained for example by subjecting the station address to an
encryption algorithm such as a randomly seeded scrambling
polynomial, whereby long length station addresses may be replaced
by changing shorter length encrypted codes. In yet another
alternative arrangement, the collision reporting packet is limited
to just the source address. The transmission of such a signal is
sufficient to cause the provision of a carrier detect signal on the
line 75 in the distribution system 30.
Returning now to FIG. 6, the collision reporting packet (CDT) 162
transmitted by station No. 3 during timing interval T3 of timing
frame FR3 is received by the distribution system 30. The receipt of
a collision reporting packet is identified by a decoding operation
within the data handling unit 76 which is then caused to transmit
during the timing interval Tl of the next timing frame FR4, a
collision acknowledgement packet (CAKT) 190, instead of a heartbeat
packet or a retransmitted information packet.
It should further be noted that if two or more stations report
collisions and transmit collision reporting packets, total signal
cancellation is not possible and some signal modulation will
therefore be received by the distribution system 30 during timing
interval T3 of timing frame FR3, and detected by a carrier detect
signal provided on the line 75 (FIG. 3). Accordingly, the
distribution system 30 is further arranged to respond to the
receipt of such carrier detect signal during such timing interval,
to transmit a collision acknowledgement packet (CAKT).
Referring briefly to FIG. 4D, the collision acknowledgement packet
190 is shown as including a preamble portion 192, a start delimiter
portion 194, a collision acknowledgement code (CAKT) portion 196
and an end delimiter portion 198.
The receipt by the stations 12 of the collision acknowledgement
packet 190 informs all user devices 16 over line 20 that a
collision has occurred. Stations 1 and 3 then terminate message
transmission, and by means of a random timer included in the user
device 16 cause a transmission retry at a later random time.
In the first timing interval Tl of the next timing frame FR5, a
heartbeat packet (HBT) 200 is transmitted by the distribution
system 30, thereby returning the network 10 to a condition when any
station 12 can commence to transmit a message, in the manner
described hereinabove.
The second reception condition at the distribution system 30,
during timing interval T2 of timing frame FR2, is that the capture
effect is not effective, for example if approximately equal
strength signals are received from two transmitting stations. This
condition may be detected at the distribution system 30 for example
by detecting that a timer set at the commencement of timing
interval T2 has expired without an end of transmission code (134 in
FIG. 4B) having been detected. Alternatively, clock samples
recovered at the distribution system 30 from the received data may
be monitored and irregularities in such monitored clock signals
used to provide a signal indicating that such clock signals must
have come from more than one station. Whatever detecting procedure
is used, if the distribution system 30 fails to detect a valid
information packet, in other words, if an indecipherable
information packet is detected, the distribution system responds by
transmitting a collision acknowledgement packet (FIG. 4D) during
the next timing interval Tl.
There will now be briefly described a second embodiment of the
invention, employing frequency division multiplexing. Referring to
FIG. 7, there is shown a local area network 10A employing a
frequency division multiplexed mode using three transmission
channels identified by transmission frequencies f.sub.1, f.sub.2
and f.sub.3. The network 10A includes a distribution system 30A and
stations 12A, identified individually as 12A-1 to 12A-N. In FIG. 7,
circuit components corresponding to those in the network 10 of FIG.
1 are identified by identical reference numbers with an A
suffix.
The frequency f.sub.1 is used for transmission from the
distribution system 30A. The frequencies f.sub.2 and f.sub.3 are
used for transmissions from the stations 12A.
The operation of the network 10A will now be briefly described. A
station 12A desiring to transmit a message, upon detecting silence
(absence of a carrier signal) on the f.sub.1 channel, sends an
information packet on the f.sub.2 channel. The distribution system
30A retransmits the received information packet on the f.sub.1
channel. The stations 12A compare the information packet received
on the f.sub.1 channel with the information packet, if any, which
they transmitted on the f.sub.2 channel. In the case of a mismatch,
a collision reporting packet is transmitted on the f.sub.3 channel
by any station 12A which detects such a mismatch.
In response to a collision reporting packet being received, the
distribution system 30A responds by transmitting a collision
acknowledge condition on the f.sub.1 channel.
Furthermore, if the distribution system 30A does not "capture" the
transmission of a particular transmitting station 12A, it may
itself detect a collision, as explained in connection with the
first embodiment, and respond by transmitting collision
acknowledgement condition on the f.sub.1 channel immediately. In
this manner all stations on the f.sub.2 channel will explicitly
detect the collision.
It will be appreciated that the described networks 10 and 10A have
the advantage that even though a transmitting station 12 or 12A
captures the transmission channel, collision with another
transmitting station 12 or 12A can be reliably reported, and
message transmission aborted for a subsequent retry.
While the forms of the invention shown and described herein are
admirably adapted to fulfill the object primarily stated, it is to
be understood that it is not intended to confine the invention to
the forms or embodiments disclosed herein, for it is susceptible of
embodiment in various other forms within the scope of the appended
claims.
* * * * *